stopWatchingPayment broken by Baron restart

[wtogami]

We thought stopWatchingPayment() may be redundant given checkPaymentExpiration()

function checkPaymentExpiration(payment) {
  var curTime = new Date().getTime();
  var expirationTime = Number(payment.created) + config.spotRateValidForMinutes * 60 * 1000;
  if(payment.status === 'unpaid' && expirationTime < curTime) {
    payment.watched = false;
    db.insert(payment);
  }
}

checkPaymentExpiration() does almost the same thing except time granularity is limited to the updateWatchListInterval.

However removing stopWatchingPayment() breaks something, suggesting USD handling is broken in the case of restarting Baron. Need to investigate.